Enterobacteriaceae (enterics)
are facultative Gram-negative rod-shaped bacteria that inhabit the
intestinal tracts of humans and many animals. Enterics that can
ferment lactose (lactose positive) are called coliforms
and usually are considered to nonnpathogenic in the intestinal tract (e.g.
Escherichia coli, Enterobacter aerogenes). (Note that
in recent years certain strains of E. coli such as O157:H7 have
been implicated in causing bloody diarrhea.) The enteric pathogens Salmonella
and Shigella are unable to ferment lactose (lactose negative). Two
types of selective and differential media are commonly employed for
isolating and identifying Enterobacteriaceae: eosin methylene blue (EMB)
agar and MacConkey agar. Lactose positive colonies are visibly
different from lactose negative colonies on both types of media.

EMB agar Plate Test:

Eosin methylene blue agar, as the
name suggests, contains the dyes eosin and methylene blue. EMB agar
is selective because the aniline dyes in this purple media inhibit growth
of Gram-positive organisms. Lactose fermenters metabolize the
lactose in the media and produce acid byproducts, causing a color change
in the colony. Thus, EMB is also a differential medium. Strong acid
production by organisms such as E. coli results in a metallic green
sheen. Weaker fermentation of lactose results in colonies with a
pinkish-purple color. Colonies of nonlactose fermenters remain
colorless, or at least are no darker than the color of the media.

MacConkey Agar Plate Test:

MacConkey agar contains lactose,
bile salts, neutral red and crystal violet. It is a selective media
because Gram-positive organisms are inhibited by the bile salts and the
crystal violet. When bacteria ferment lactose and produce enough
acid products to reduce the pH below 6.8, the neutral red turns from
colorless to red. Thus, MacConkey is a differential media on which
lactose fermenting colonies appear red (or pink). Nonlactose
fermenters are colorless.